288 — The American Naturalist. j [March, 
Marine Biological Laboratory at Woods Holl, towards the end A that 
month, I had no difficulty in obtaining large quantities of ova in the 
earliest stages of development, and I succeeded in keeping the embryos 
alive until the end of August, by which time they had developed into 
Scyphistomas with about twenty tentacles. 
The developmental history as I observed it differs in so many points 
from what Agassiz has described, as well as from the observations of 
other authors upon European forms, that I wish to postpone a detailed 
account of my observations until I shall have had an opportunity of 
studying for comparison the embryology of Aurelia flavidula, which I 
hope to accomplish during the coming summer. In the meantime I 
wish to record here briefly some of the more important facts which I 
have been able to establish. 
e segmentation is practically regular (though the relative size of 
the first-formed spherules may, vary considerably), and results in the for- 
mation of a blastula. Certain cells then migrate into the blastoccele, 
and arrange themselves as an incomplete layer below the cells which 
remain at the surface, and at the same time an opening appears at one 
pole of the embryo. This pseudogastrula is, however, very transient, 
The immigration of cells continues, being apparently multipolar in its 
distribution, and the opening closes up. Eventually a solid planula or 
sterrula results, consisting of an external layer of columnar cells anda 
central mass in which the cell outlines cannot be made out in sections. 
In this condition the embryos may persist for some time, swimming 
about actively. From time to time, however, some settle down to the 
bottom of the vessel in which they are contained, and enclose them- 
selves in a circular plano-convex cyst. I found a few free-swimming 
embryos, out of the many hundred which I examined, which had 
developed a mouth and a central cavity, and possessed a rudiment of a 
single tentacle, but their further development I was not able to observe. | 
It is certain that the majority encysted themselves in the manner 
described, but it is of course possible that this may be due to unsatis- 
factory conditions of life, though the fact that large numbers of the 
encysted form developed into Scyphistomas argues against such an idea. 
While within the cyst, the hollowing out of the central mass 
the formation of the endoderm take place. The encysted state lasts 
for several days, but finally the embryo emerges from the cyst 
a circular aperture in the center of the free convex surface of the cyst, 
formed apparently by solution, as I never saw any ragged edges to "3 
opening. I could not at first believe that the encystment was a stage m 
the development ; it seemed rather to mean the death of the embryos- 
